The present invention relates generally to the field of voltage controlled oscillators, and specifically to a method of automatically tuning a voltage controlled oscillator over a wide frequency range.
Wireless radio communications systems transmit voice and other data between fixed transceivers and mobile radio communications terminals via the propagation of radio frequency (RF) electromagnetic waves. Essential to the functionality of such a wireless communication system is the stable and accurate generation of oscillating electrical signals. One circuit widely used to generate such oscillating signals is a phase-lock loop (PLL). A PLL is an electrical circuit that generates an oscillating output signal that has a constant phase relationship with an oscillating input signal. By utilizing a highly stable and accurate source, such as a crystal oscillator, to generate the oscillating input signal, and various frequency multipliers and dividers, a stable and accurate oscillating output signal can be generated across a range of frequencies.
A critical component of a typical PLL is a Voltage Controlled Oscillator (VCO). The VCO generates an oscillating signal at its output, the frequency of which is responsive to a voltage level applied at its input. In the PLL, the voltage input, referred to herein as the tuning voltage, is a function of the phase/frequency error between the output of the VCO and the desired oscillating signal. The VCO thus generates an oscillating signal at a frequency that varies over a finite range, corresponding to variations in the tuning voltage over a corresponding finite range. The specific parameters of this voltage/frequency relationship depend upon the design of the VCO, the values of electrical components that comprise the VCO, ambient temperature, and other effects as are widely known in the electronics arts. Ideally, if plotted on a voltage/frequency axis, the relationship would define a generally linear curve with positive slope, i.e., an increase or decrease in the tuning voltage causes a corresponding increase or decrease in the frequency of the oscillating signal generated by the VCO. Such a voltage/frequency curve is referred to herein as an operating curve of the VCO.
To expand the frequency range of a VCO, it is known to selectively couple frequency altering components, such as for example capacitors, varactors, FET transistors, and the like, to the resonant circuit in the VCO. This alters the range of frequencies generated in response to the tuning voltage, in effect establishing a new operating curve for the VCO. For example, it is known to couple various capacitors to a VCO via a programmable switching matrix such that, by selectively configuring the switches, a plurality of overlapping frequency ranges for the VCO may be selected. This ensures that the VCO may be calibrated to compensate for deviations in operating its frequency range due to manufacturing process variations or other parasitic effects, by choosing a VCO operating curve to encompass the desired range of VCO operation. This calibration, also known as VCO trimming, generally occurs in the factory upon manufacture of the integrated circuit containing the VCO, such as for example, by burning fuses or by programming a particular value in a register, the contents of which control the switches connecting the transistors to the VCO oscillator.
Factory tuning of a VCO to a particular operating curve is a time-consuming, and thus costly, part of the manufacturing process. Additionally, once thus tuned, the VCO is limited to a single, finite range of operation, as it is restricted to a single operating curve.
The present invention entails a self-tuning voltage controlled oscillator circuit. The VCO produces an oscillating signal at its output, the frequency of which is responsive to a tuning voltage its input. At least one frequency altering device is selectively coupled to the VCO and is operative to alter the relationship of the tuning voltage to the VCO output frequency, thereby establishing at least two operating curves describing the relationship of the VCO tuning voltage to the VCO output frequency. The VCO is switched between the operating curves by a control circuit. In one aspect of the invention, the control circuit monitors the tuning voltage and generates outputs operative to selectively couple the frequency altering device(s) to the VCO based only on the value of the tuning voltage. In another aspect, the control circuit receives a plurality of reference voltages establishing a plurality of switching thresholds and outputs switching signals based on a comparison of the VCO tuning voltage to the plurality of switching thresholds, and on its previous outputs.